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OFJ Field Journal from Lou D'Amario - 11/13/95

SOME TWEAKS ARE BETTER LEFT UNDONE

Jupiter Orbit Insertion (JOI) Tweak

I discussed the JOI tweak (the late update to the size of the JOI maneuver) in some detail in my last journal entry. The next day, the Navigation Team was told that the Project was seriously considering eliminating the JOI tweak (see my last journal for an explanation about tweaks). This was disappointing, because the JOI tweak is an important part of the overall navigation strategy for the Jupiter encounter. The problem was that the Flight Teams were very busy changing the spacecraft sequences because of the tape recorder problems. There didn't appear to be time for the Orbiter Engineering Team and the Sequence Team to run through a test and training exercise for their parts of the JOI tweak development before arriving at Jupiter.

By Monday November 6, I was able (with the assistance of Mike Wilson and Chris Potts, who also work on the Navigation Team as maneuver analysts) to show what would happen if we eliminated the JOI tweak. The first JOI- correction maneuver at JOI plus one day (called OTM-1, the first Orbital Trim Maneuver) could get much larger, calling for more propellant. That was not good, but there was a bigger problem. If for some reason we were unable to perform OTM-1, the next maneuver (OTM-2) could get so large (in some situations) that there might not be enough propellant left to finish the full orbital tour! This became a real dilemma.

On Tuesday, I suggested a possible solution to Ralph Reichert, the Engineering Office Manager: scratch the JOI tweak test and training exercise, and keep the JOI tweak in the plan, but only on a so-called "best efforts" basis. In other words, the Flight Team would try to do the work required for the JOI tweak in the time we have. Then, if we did manage to get ready for the tweak (and the JOI tweak was judged to be desirable), the tweak commands would be sent to the spacecraft. On the other hand, if the work wasn't finished in time, no commands would be sent, and we would accept the consequences. What made this strategy feasible was that the exact same work needed for the JOI tweak had, in fact, already been done once a few months ago after the Orbiter Deflection Maneuver; it just hadn't been done on the short timeline of the JOI tweak. As of today, the JOI tweak is still in the plan (and we have our fingers crossed).

Arrival Day Telecom Meeting

On November 6, I attended the Arrival Day Telecom Strategy Meeting. This meeting was about how to configure the spacecraft and the DSN (Deep Space Network) ground antennas to make it as likely as possible that we will be receiving telemetry (data sent from Galileo) on December 7, the day that Galileo arrives at Jupiter. There were no major effects on navigation plans (we lost one tracking pass before the Io flyby, and the backup "uplink window" (the time period where we send commands up to the spacecraft) for the JOI tweak was moved later).

Trajectory Change Maneuver 27 (TCM-27)

Starting on November 8, the Navigation Team had one regular working day (8 hours) to finish working out exactly how great a change in velocity would be needed for the trajectory change maneuver (TCM-27). This maneuver improves our aim for when Galileo flies by Io. This work involved the following steps:

First, the Orbit Determination (OD) Group "generates a solution" (more on this later) for the spacecraft trajectory (the spacecraft's path) and the predicted Io flyby conditions using the most recent tracking data available. Then the Trajectory Analysis (TRAJ) Group checks the OD solution by mathematically "moving" the spacecraft all the way to Io closest approach to see if the Io flyby will match the predictions. Since we never get an exact match, the Maneuver Analysis (MNVR) Group then calculates how we have to maneuver the spacecraft so that it is perfectly on target (specifically, they calculate a velocity vector that will correct the miss in the Io flyby conditions). Once again, the TRAJ Group checks the spacecraft's trajectory ---but this time with the maneuver added in. If everything checks out, the Group sends the maneuver information to the Orbiter Engineering Team (OET); it' s the OET that will translate the Navigation team's request into actual commands for the spacecraft's thrusters.

Finally, the results of all this work were presented to the Project managers. Here's a summary of the results. The desired Io closest approach altitude is 1000 km, and the desired closest approach time is at 17:45:44 UTC on December 7, 1995 (UTC refers to the 24-hour clock time at Greenwich, or 5:45:44 PM at Greenwich. At JPL, that will be 9:45:44 on the morning of December 7). Without TCM-27, Galileo would miss its aimpoint at Io by a predicted 84 kilometers in altitude (a large error), arriving 5 seconds early. To correct these errors, the spacecraft velocity would have to be changed by 0.16 meters/second (a small maneuver). Then, at the conclusion of the presentation, the Navigation Team recommended that TCM-27 be canceled! Why did we do that? There is some additional information about the maneuver design process that I haven't mentioned yet.

When the OD Group "solves" for the trajectory, what they are really doing is finding the trajectory that best "fits" the radio tracking data. In other words, orbit determination is basically finding the trajectory that differs as little as possible from the observed tracking data. When the OD Group determines the best trajectory fit, they are also able to say how "good" the fit is - i. e., how small the differences from the observed tracking data are. For TCM-27, the errors in the Io flyby were small relative to these differences -- we didn't know the miss at Io accurately enough to say that correcting the errors at TCM-27 would actually improve the trajectory. In recommending to cancel TCM-27, we were saying that we needed to wait until the spacecraft got closer to Io and Jupiter so that the "goodness" of the trajectory solution would improve. The Project accepted the Navigation Team recommendation, and TCM-27 was canceled.

The next maneuver, TCM-28, is scheduled for November 27 (Io minus 10 days). The design of TCM-28 starts on November 18. We will have 10 more days of tracking data to reduce the uncertainties of the trajectory (i. e., improve the "goodness" of the fit). I expect that the Navigation Team will not recommend canceling TCM-28.

Non-Work Stuff

So far, the increased level of activities at work have not disrupted my exercise schedule. I am still riding my bike or walking every morning for 30- 35 minutes; on Saturday and Sunday I go for a longer bike ride (about an hour). So far, the Navigation Team has not had to work nights or weekends, but that will change for the upcoming maneuvers and the JOI tweak. We will be working this coming weekend and the weekend after Thanksgiving. And there are some all-niters coming. At home, my wife Maria and I are planning to have some work done on our kitchen - new floors, countertops, appliances, lighting etc. She will have to assume responsibility for negotiating with the contractors for this work until after Jupiter arrival - I'm just too busy right now. I plan to take some vacation time at Christmas. My mother is coming Barbara, will also be at our house for Christmas.

 
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